I compared my 71 bottom frame with a 72 bottom, they looked the same to me. I used the 72 bottom b/c i had a few broken springs in mine. Now, im real curious about the backs. i will say that when i mounted my 6way brackets on the 72 frame (which came with 6ways as well) my tracks bind up ever so slightly. i'll have to pull it back out and check it in the spring.